The role of baroreceptor inputs in the modulation of sympathetic nerve activity both under control conditions and under conditions known to alter baroreceptor characteristics will be examined. Sympathetic nerve activity will be studied at three different levels. In the first phase of the study, activity of single afferent fibers from sympathetic nerves will be measured in anesthetized rabbits. In the second phase activity from whole sympathetic nerves will be recorded in anesthetized rabbits. In the final phase chronic recordings of sympathetic nerve activity will be made in conscious, unanesthetized rabbits. The studies will primarily be concerned with renal sympathetic nerves but cardiac and muscle sympathetic nerves will also be included. In all phases ofthis study, surgically implanted perivascular balloons will be used to briefly raise and lower blood pressure to determine the relationship of sympathetic nerve activity to mean arterial blood pressure. Curve characteristics including the blood pressue at which activity is minimal, the slope on gain and the range o activity for the barofeflex relationship will be determined. Prolonged alteration of resting blood pressure will be used to study the effects of acute barcreceptor resetting on baroreflex modulation of sympathetic activity. Single fiber efferents will be examined to determine whether whole nerve activity reflects responses in a homogenous or heterogeneous population of individual efferents and thus to interpret the response of whole nerve sympathetic activity. The contribution of afferent input from high and low pressure baroreceptors will be studied by selective aortic and carotid sinus denervations. More specific knowledge of the baroreceptor involvement in modifying sympathetic outflow is important to understanding the neural control of the circulation during prolonged blood pressure alterations, such as duing therapeutic drug treatment.